Abstract
Oil-pressboard insulation in transformers is prone to surface tracking or creeping discharge during operation. The oil-pressboard interface is believed to be one of the weakest points in the insulation system. In this paper, AC flashover characteristic at the triple junction of the oil-pressboard interface under a slightly inhomogeneous electric field has been studied. The results showed that the creeping discharge always started at the triple junction, then developed along the pressboard (PB) surface, and finally reached the opposite electrode. The reason was that the triple junction was subjected to the maximum electric field strength. The field strength corresponding to the breakdown of the pure oil gap (POG) was decreased with the increment in the gap distance between electrodes due to the volume effect. As a comparison, the field strength corresponding to the breakdown for PB only changed slightly with the gap distance. Besides, the breakdown electric field of both POG and PB hardly changed when the electrode diameter increased. The above results could be applied to the inverse calculation and estimation of the flashover voltage levels under longer gap distances with any electrode diameter. Moreover, the relative position of the oil gap and PB could influence the flashover voltage levels. The breakdown voltage with PB directly contacting the ground electrode was 10% higher than that with PB directly contacting the high voltage electrode. This could be explained by the different charge behavior. The obtained results could provide a reference for the insulation margin design in ultra-high-voltage transformers.
Highlights
Oil-impregnated pressboard (PB) was commonly used to partition the large oil gaps to enhance the overall dielectric strength of the composite insulation system in transformers
It is believed that the oil-PB interface becomes the electric weak link of the composite insulation system [1]
In the experiment to study the effect of relative location of pure oil gap (POG) and PB, the electrode gap was set at a constant distance of 30mm
Summary
Oil-impregnated pressboard (PB) was commonly used to partition the large oil gaps to enhance the overall dielectric strength of the composite insulation system in transformers. It is believed that the oil-PB interface becomes the electric weak link of the composite insulation system [1]. According to the CIGRE report, insulation failures within the winding coil, on-load tap, and connecting device comprise most of the malfunction in transformers [2,3]. In these weak areas, the insulation of oil-immersed transformers requires compatibility with the formation of the oil path for cooling and has a complicated structure with static shields and L-shaped PB barriers. A high creepage electric field is produced on the surface of the solid insulation. An eventual flashover may follow to make a total breakdown leading to a catastrophic transformer failure [5]
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